Propulsion of marine vessels by electric drives is an increasing trend in the marine industry. For medium and small vessels, which have installed power of less than around 10 MW, the distribution voltage is expected to remain at 690V AC. For larger vessels however, the operating voltages are much higher, e.g. 3.3 kV or 6.6 kV, as these lead to significant advantages (better efficiency and less cables for example). Generally, these electric drives can be controlled through AC/AC converters such that the output AC waveform is adjustable. Some AC/AC converters are AC/DC-AC converters, so that the input AC waveform is converted to DC (via a DC-link) before being converter to the output AC waveform.
To achieve operation of these electric drives from a high voltage input, multilevel inverters are preferred. These inverters can be classified into three main categories: neutral point clamped (NPC) inverters; flying capacitor inverters, and cascaded multilevel inverters. For voltage levels higher than 3.3 kV, circuits using either NPC or flying capacitor inverters become increasingly complex. The split DC capacitors voltage balance control can also increase in complexity. The non-modularised design is another disadvantage of both the NPC and flying capacitor inverters since these systems have very low redundancy and any single component failure may lead to a whole system failure. The cascaded multilevel inverter do not suffer these deficiencies and has become a popular topology for voltage levels higher than 3.3 kV.
There are many advantages of a cascaded H-bridge (CHB) multilevel converter compared to, for example, a neutral point clamped (NPC) multilevel converter. These include a modularized design with high redundancy, a design which is easy to extend to higher voltages, a low
      dv    dt    ,an easy DC-link voltage balance control of modules etc. However, each cell of a CHB is a single-phase converter, where the instantaneous output power is not constant as it would be with a three-phase balanced NPC type converter. The power has 2nd order load current frequency oscillation, which can lead to the requirement of a large DC-link capacitance to smooth out the DC-link voltage fluctuation. The large size DC-link capacitor bank results in a large and heavy CHB cell.